Mobile aerial lift vehicles are in common use for lifting workman at a job site to a height where they can work on elevated structures. Typically, these vehicles are four-wheeled, and have an elevating assembly mounted on the vehicle chassis for raising and lowering the work platform.
In addition, these vehicles are provided with ground-engaging outriggers at each of the four corners of the chassis. Typically, the outriggers each include a hydraulic cylinder to lower or raise the outrigger pads relative to the chassis. The outriggers are used, even when the vehicle is on level ground, to rigidify the system and prevent any movement of the elevated work platform that might be caused by "bounce" of the pneumatic tires during work operations.
The outrigger system is also used to level the entire vehicle if the ground slopes at the job site, so that the work platform can be raised in a purely vertical direction. Prior to the present invention, the operator leveled the vehicle by watching the bubble, or bubbles, of a level indicator and by individually controlling the amount of extension of the four outrigger hydraulic cylinders. If the ground slopes in a single direction, two of the corners of the chassis will be lower than the uphill corners and will have to be lifted by the hydraulic cylinders at the downhill corners. If the ground slopes in two directions, three of the corners of the chassis will have to be raised to the level of the uphill corner. Since the chassis is rigid, raising of one corner of the frame will cause undesirable rocking of the vehicle about a diagonal axis through the outrigger being extended, unless the other two outrigger pads are simultaneously controlled by the operator so that they are maintained in ground contact. Manual control of the leveling process is time consuming and requires considerable operator attention.